In a conventional dump truck, material such as dirt, sand or gravel is transported from one location to another and then "dumped" on the ground. The material is dumped by elevating one end of the dump bed such that the material slides out the other end. The dump bed is elevated by means of a hoist operated by a hydraulic pump. The hydraulic pump is powered by engine power diverted to the pump by means of a power take-off ("PTO"). The PTO is a supplementary mechanism that enables engine power to be used for auxiliary systems on the dump truck, such as the hydraulic pump.
In order to raise the dump bed, the dump truck driver or operator engages the PTO, which diverts engine power to the hydraulic pump. The driver then engages the hydraulic pump which operates the hoist and lifts the dump truck bed. The PTO and the hydraulic pump are engaged by means of a controller generally located in the cab of the truck.
In normal operation, once the dump truck driver has dumped the material at a job site, he is to lower the bed with the hydraulic pump and disengage the PTO before traveling down the highway. It is not uncommon, however, for a driver to leave inadvertently the bed raised or the PTO engaged. In the worst case scenario, the height of the truck with the bed raised may exceed the height of a bridge or other overhead object. Should a dump truck traveling with the bed in the raised position strike a bridge, the damage to the truck, driver and bridge can be significant. Similarly, if the PTO is left engaged it can easily overheat, causing damage to the truck and, on occasion, injury to the driver or bystanders.
Currently known devices disclosed in the literature to disengage automatically the hydraulic pump and the PTO are not adequate for normal dump truck operation. For example, Japanese Patent No. 60-151139 to Toukiyuu Shiyariyou Seizou K.K. discloses a dump truck operation warning device. The PTO is automatically cut off and a warning device is activated after the hydraulic pump has been operated for a certain amount of time while traveling with the PTO engaged.
The disadvantage of this system is that it limits the ability of the dump truck operator to perform certain low speed operations with the bed raised. These operations would include, for example, gravel spreading. A system that is time controlled, i.e., based on the amount of time the hydraulic pump has been operated or the PTO has been engaged, is incompatible with low-speed gravel spreading because it would interrupt the job by automatically lowering the dump bed.
Despite the various efforts found in the prior art, there remains a need for a speed or a gear controlled dump truck system in order to increase safety while at the same time permit low speed operations. Such a system would ensure that the dump truck could not obtain high speed or high gear with the bed raised while maintaining the versatility needed for job site operations.